this post was submitted on 14 Apr 2026
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TIL that 27" tubes can work on a 29" rim. (pawb.social)
submitted 2 weeks ago by OwOarchist@pawb.social to c/bikewrench@lemmy.world
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When my girlfriend suggested we take the tubes off an old 27" bike to fix the flats on my 29", I thought she was being crazy and overly cheap. No way it would work. Surely I'd have to go out and buy some 29" tubes.

But fuck me sideways, it did work, and I'm currently riding just fine with 27" tubes in my 29" tires. Wasn't even that hard to get them on.

So I guess that's just a PSA for everybody ... apparently, tube sizes aren't really rules, they're more like guidelines.

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[–] pr06lefs@lemmy.ml 4 points 2 weeks ago* (last edited 2 weeks ago)

I think this bit of insanity is due to 27 referring to the outer diameter of the tire rather than the rim diameter. "27" appears to be a road bike standard as tire sizes are around 1 inch on that Schwalbe chart. If you have a 630mm rim and a 1.1 inch tire, 630 / 25.4 + (1.1 * 2) = 27.0 for the outer diameter.

On the other hand, "29 inch" is a mountain bike standard with a tire size ranging from 2 to 2.4 inches or so. For a 622mm rim and 2.2 inch tire you get 622 / 25.4 + (2.2 * 2) = 28.8 outer diameter.

If the OP has a "29" inch tire, that's a mountain bike. And I suspect their "27" inch tire bike is also a mountain bike, which is actually a "27.5" size which is a mountain bike standard, with a 584mm rim diameter. If that's the case they would have had to stretch the tube a bit to get it on the 622mm rim of the 29er, but at least that tube would be intended for a mountain bike sized tire, say 2.4 inches.